Plastic cycling water bottles (bidons) are notorius good growing ground for mould and fungus. However, I am not particularly upset about it, partly because I am not too sensible to dirt, but mostly because I don't recommend the use of cycling bidons in the first place. Ordinary plastic 1L bottle is almost twice lighter then 0.7 L bidon and has more volume. It also has simpler mouthpiece that doesn't attract mould.
But if, for whatever reason, you insist on using cycling bidons, I have a simple tip how to clean it.
You don't need any special chemicals, heat treatment or dish washer. Just put a paper towel or a few pieces of (unused) toilet paper in the bottle and stir and rub with a long stiff stick. Instead of a stick you can use the hooked end of a spoke.